Slow variations of the reactivity in pressurized-water nuclear reactors consist of an adjustment of the concentration of a soluble poison in the coolant of the primary circuit. These reactivity variations are essentially due to depletion of the fuel during the cycle, to poisoning of the core by the production of fission products such as xenon, or to changes in the temperature of the coolant between cold stopping and hot stopping conditions. The soluble poisons commonly used may be gadolinium or boric acid salts.
It is conventional to adjust the concentration of the soluble poison in the coolant by injecting a concentrated solution of poison or by diluting the coolant. This latter operation leads to the formation of a large volume of effluents which must be processed before reuse in the primary circuit or before disposal.
To avoid having to process these effluents before their reuse or disposal, it has been attempted to reduce the volume of effluents obtained during adjustment of the concentration of the poison in the coolant. For example, in French Patent FR-A-1,598,608, in order to modify the concentration of soluble poison in the coolant, use is made of ion exchange resins loaded with poison and whose reserve capacity during flowing of the coolant is modified by varying their operating temperature, i.e., that of the coolant. Fixing of the poison on the ion exchange resins takes place at a temperature of the order of 0.degree. to 40.degree. C., and its release occurs at a temperature of the order of 60.degree. to 90.degree. C.
Thus, the effluents are stored in the resins and are not discharged. There is therefore no problem as to the reprocessing of these effluents.
French Patent FR-A-1,477,058 also discloses a method for processing cooling fluid, in which the concentration of dissolved boric acid is adjusted by bringing the cooling fluid into contact with an ion exchange resin which retains the boric acid, the resin is regenerated with a caustic alkali which removes the boric acid from the resin, and the boric acid and the regenerating agent are separated by electrodialysis.
However, although the methods described in these patents do not have the drawback of the formation of a large volume of effluents, they have two other main drawbacks. In fact, these methods permit only slow variation of the soluble-poison concentration and adjustment of only certain concentrations of poison situated in a range which is a function of the resins.